Silica sol casting refers to a casting method in which a pattern is made of fusible materials, several layers of refractory materials are coated on the pattern to form a shell. After the pattern is melted, it is baked at a high temperature then poured.

The process of silica sol casting:

1 manufacturing mold

The mold is used to press the shape of the workpiece. In order to ensure the quality of the wax mold, the mold must have high dimensional accuracy a small surface roughness value. When the casting precision is high the production batch is large, it can be made by directly pouring fusible alloy, epoxy resin, gypsum on the master mold; when the casting precision is high mass production, the pressure is generally made of steel, copper alloy, Aluminum alloy is made by cutting, most enterprises use aluminum alloy molds.

2 Pressed wax model

The processed wax paste is pressed into the mold cavity, the ice water is condensed by the ice water machine, then taken out. After being placed in the pool for a period of time, after trimming inspection, a single wax mold is obtained. Wax mold can be understood as a pressure casting part.

2.1 Pressure wax process parameters

Room temperature: 25℃±2℃, wax bath temperature: 56~60℃, wax injection nozzle temperature: 56~60℃, wax injection pressure: 2.5~15MPa (specifically should be adjusted according to the size thickness of the workpiece), pressure holding time: Several seconds to tens of seconds (adjusted according to the size of the workpiece wall thickness), the cooling water temperature: 9℃±2℃.

2.2 Operation precautions

Check the model of the mold before pressing the wax, open the mold to clean wipe the mold cavity, remove the residual wax on the mold. Only use a bamboo knife. Metal objects are allowed to contact to prevent damage to the mold surface. Blow the cavity of the mold with compressed air spray the mold release agent to close the mold. The pressing parameters cannot be easily changed during the wax mold pressing process. If the pressed wax mold has partial bulging, the wax mold has missing corners, the deformation cannot be repaired, the wax mold with size problems should be treated as scrap.

3 repair mold

The quality of wax mold, surface quality, degree of deformation determine some important parameters of the product. Therefore, the wax mold workshop needs constant temperature control to minimize the deformation of the wax mold. At the same time, the surface of each wax mold needs to be inspected. The scars should be washed away, the burrs, flashes parting lines of the wax parts should be removed at the same time, the minor defects should be repaired.

4 Combination wax model (group tree)

After the mold is repaired, in order to improve the productivity of the qualified single wax mold, the single wax mold is welded on the pre-made wax public casting system to form a wax module. Usually 2 to 80 wax patterns can be welded on a wax module. The tree grouping scheme determines the product yield the utilization rate of molten steel. The tree grouping scheme should be carefully considered when designing, at the same time, whether the tree grouping scheme is convenient for wax recovery reuse.

 5 Clean the wax mold

The wax module after the tree formation can be cleaned after standing for more than 45 minutes. Before the shell slurry is made, the wax module after the number of sets must be cleaned to remove the wax dust grease on the surface improve the adhesion of the shell. The module is immersed in the cleaning solution reciprocated three times for a total of about 4 seconds. After taking it out, the module can be dried with compressed air then the shell can be sizing.

6 manufacturing shell

After dipping the paint outside the wax mold, it is placed in the shell-making room to dry. The surface layer adopts the method of natural drying, the transition layer the back layer adopt the rotating air drying method to accelerate the drying. Repeat the coating 5-7 times (small castings) 5 to 6 layers, 6 to 9 layers of large castings. Use finer sand for the surface layer, slightly thicker sand in the transition layer, thicker sand in the latter layers), until the coating forms a hard shell of 7-15mm. A hard shell mold with sufficient strength is called a mold shell.

6.1 Ratio

According to different products, different viscosity values should be used. If the shape structure are complex, the coating should be thinner. The shape is simple there are large area flat products, the coating should be thicker.

6.2 Control requirements

The temperature between the shells is 25℃±2℃, the humidity between the surface layers is 55%~70%, the transition layer is 50%~65%, the back layer is 40%~60%. When the surface coating is new material, the stirring time is ≥24h, part of the new material is stirred for ≥12h, when the transition layer back layer are new material, the stirring time is ≥12h, the part of the new material is stirred for ≥6h.

6.3 Key points of sizing operation

Slowly immerse the cleaned module into the paint bucket at a certain angle, rotate it slightly, take out the module at a slight speed, drip off the excess paint, blow the air bubbles attached to it with compressed air (for fonts grooves) If there is any place there is no paste, use a brush to repair it locally), keep rotating the module, enter the sand shower during rotation, cover the surface of the module with even sand, then hang it on the turnover trolley dry. The sanding of the transition layer the back layer is carried out in the same way. After the last layer of the back layer is dried, the final sealing slurry is dried.

6.4 Shell making drying time control

The drying time of the surface layer is 6~8h, the drying time of the transition layer is ≥10h, the drying time of the back layer is ≥12h, the drying time of the sealing layer is ≥14h. The drying time of different sands is also different. It can be adjusted appropriately according to the drying situation. Sand loss cracks indicate that the shell is too dry, so the drying time should be shortened appropriately.

7 Dewaxing wax treatment

Pour the mold shell into the steam dewaxing axe to melt the wax mold flow out below. It is separated by the water separator to collect the wax liquid, put it in the water removal bucket for further wax-water separation removal of impurities in the wax. After hours, put the wax liquid through the filter into a static bucket for 6 to 8 hours, finally put the wax liquid in an incubator for heat preservation for reuse. The shell after the wax mold flows out is the casting mold.

Note: (1) The time the shell exiting the shell making room to entering the dewaxing axe should exceed 1.5 minutes, preferably within 1 minute; (2) After entering the dewaxing axe, the pressure in the dewaxing axe should be 13 The minimum value can reach 0.6MPa within a second; (3) Except for the mixing temperature of the bucket, the temperature is 110~120℃, the temperature of the static bucket is less than 90℃, the wax is easy to age if the temperature is too high, the temperature of the incubator is controlled at 54~62℃.

Type 8 shell roasting, pouring, falling sand cleaning

The dewaxed shell should be roasted during pouring. Put the shell into an oil furnace electric furnace at 1100°C for 50 minutes. The main purpose is to remove the residual wax liquid after dewaxing improve The strength air permeability of the shell can prevent the shell being deformed broken during casting. Another extremely important factor is to reduce the temperature difference between the castable molten steel the shell to reduce the shrinkage of the workpiece. When the molten steel is melted stabilized can be poured, the shell that has reached the baking holding time in the roasting furnace can be poured. After the casting is condensed, the mold is destroyed, the gating system is removed, the burrs are cleaned shot blasting is performed to obtain the target casting. The surface of stainless steel castings usually needs to be pickled passivated.

Concluding remarks

Reasonable process parameters strict production process control are the prerequisites for silica sol casting. According to different product data, it needs to be adjusted according to the actual situation, such as the viscosity of the surface coating, which needs to be adjusted according to the product size complexity , Reasonable selection requires craftsmen to have certain work experience.